The polymorphic oxidation of beta-adrenoceptor antagonists

Induction of CYP2D6 in pregnancy

Expression of the drug-metabolizing enzyme cytochrome P4502D6 (CYP2D6) is predominantly under genetic control, and enzyme-inducing drugs have little influence on its activity. We studied the activity of CYP2D6 during pregnancy. One hundred forty pregnant women were genotyped for CYP2D6. Seventeen of them (four poor metabolizers, seven heterozygous extensive metabolizers, and six extensive metabolizers) were phenotyped with dextromethorphan in late pregnancy and 7 to 11 weeks after parturition. During pregnancy the dextromethorphan/dextrorphan metabolic ratio was significantly reduced (p = 0.0015) among homozygous and heterozygous extensive metabolizers, indicating increased CYP2D6 activity. In contrast, poor metabolizers showed an increased metabolic ratio during pregnancy. These results are consistent with previous findings of a marked increase in metabolism of the CYP2D6 substrate metoprolol during pregnancy. Both studies indicate an increase in CYP2D6 activity during pregnancy, which may be caused by an induction of the CYP2D6 enzyme.

A rapid and sensitive electron-capture gas chromatographic procedure for analysis of metoprolol in rat brain and heart

A procedure for analysis of metoprolol-utilizing extraction followed by derivatization with pentafluoropropionic anhydride and analysis on a gas chromatograph equipped with a fused silica capillary column, an electron-capture detector and a printer/integrator is described. Propranolol was carried through the procedure as internal standard. The pentafluoropropionyl derivative of metoprolol yields a sharp peak on the gas chromatograph, and the structure of the derivative was confirmed using combined gas chromatography-mass spectrometry. The analytical method is linear, sensitive and reproducible and has been applied to analysis of metoprolol in brain and heart from rats treated with metoprolol intraperitoneally. Pretreatment of the rats with the antidepressant desipramine prior to metoprolol administration resulted in a marked increase in levels of metoprolol in both brain and heart, indicating a pharmacokinetic drug-drug interaction between desipramine and metoprolol.

Disposition and bioavailability of the beta 1-adrenoceptor antagonist talinolol in man

In an open randomized crossover study, the pharmacokinetics and bioavailability of the selective beta 1-adrenoceptor antagonist talinolol (Cordanum--Arzneimittelwerk Dresden GmbH, Germany) were investigated in twelve healthy volunteers (five female, seven male; three poor and nine extensive metabolizers of the debrisoquine hydroxylation phenotype) after intravenous infusion (30 mg) and oral administration (50 mg), respectively. Concentrations of talinolol and its metabolites were measured in serum and urine by HPLC or GC-MS. At the end of infusion a peak serum concentration (Cmax) of 631 +/- 95 ng mL-1 (mean +/- SD) was observed. The area under the serum concentration-time curve from zero to infinity (AUC0-infinity) was 1433 +/- 153 ng h mL-1. The following parameters were estimated: terminal elimination half life (t 1/2), 10.6 +/- 3.3 h; mean residence time, 11.6 +/- 3.1 h; volume of distribution, 3.3 +/- 0.5 L kg-1; and total body clearance, 4.9 +/- 0.6 mL min-1 kg-1. Within 36 h 52.8 +/- 10.6% of the administered dose was recovered as unchanged talinolol and 0.33 +/- 0.18% as hydroxylated talinolol metabolites in urine. After oral administration a Cmax of 168 +/- 67 ng mL-1 was reached after 3.2 +/- 0.8 h. The AUC0-infinity was 1321 +/- 382 ng h mL-1. The t 1/2 was 11.9 +/- 2.4 h. 28.1 +/- 6.8% of the dose or 55.0 +/- 11.0% of the bioavailable talinolol was eliminated as unchanged talinolol and 0.26 +/- 0.17% of the dose as hydroxylated metabolites by kidney. The absolute bioavailability of talinolol was 55 +/- 15% (95% confidence interval, 36-69%).(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of CYP2D6-dependent metabolism on the steady-state pharmacokinetics and pharmacodynamics of metoprolol and nicardipine, alone and in combination

1 The metabolism of metoprolol depends in part on the genetically determined activity of the CYP2D6 isoenzyme. In vitro studies have shown that nicardipine is a potent inhibitor of CYP2D6 activity. Since the combination of metoprolol and nicardipine is likely to be used for the treatment of hypertension, we examined the interaction between these two drugs at steady-state. 2 Fourteen healthy volunteers, seven extensive and seven poor metabolisers of dextromethorphan were studied in a double-blind, randomised cross-over four-period protocol. Subjects received nicardipine 50 mg every 12 h, metoprolol 100 mg every 12 h, a combination of both drugs and placebo during 5.5 days. Steady-state pharmacokinetics of nicardipine and metoprolol were analyzed. Beta-adrenoceptor blockade was assessed as the reduction of exercise-induced tachycardia. 3 During treatment with metoprolol, alone or in combination with nicardipine, its steady-state plasma concentrations were higher in subjects of the poor metaboliser phenotype than in extensive metabolisers. Beta-adrenoceptor blockade was also more pronounced in poor metabolisers than in extensive metabolisers of dextromethorphan during treatment with metoprolol alone or in combination with nicardipine (24.0 +/- 2.4% vs 17.1 +/- 3.5% and 24.1 +/- 2.5% vs 15.4 +/- 2.7% reduction in exercise trachycardia, respectively, P < 0.01 in each case). 4 Nicardipine produced a small increase in plasma metoprolol concentration in extensive metabolisers from 35.9 +/- 16.6 to 45.8 +/- 15.4 ng ml(-1) (P < 0.02), but had no significant effect in poor metabolisers. However, nicardipine did not alter the R/S metoprolol ratio in plasma 3 h after dosing, the plasma concentration of S-(-)-metoprolol 3 h after dosing or the beta-adrenoceptor blockade produced by metoprolol in subjects of both phenotypes. The partial metabolic clearance of metoprolol to alpha-hydroxy-metoprolol was not altered significantly in extensive metabolisers. Plasma nicardipine concentration and beta-adrenoceptor blocking effects did not differ between the phenotypes and were not influenced by metoprolol. We conclude that beta-adrenoceptor blockade during repeated dosing with metoprolol is more pronounced in poor than in extensive metaboliser subjects, that nicardipine decreases a CYP2D6-independent route of metoprolol elimination but does not increase beta-adrenoceptor blockade during repeated dosing with metoprolol.

Enantioselective bioanalysis of beta-blocking agents: focus on atenolol, betaxolol, carvedilol, metoprolol, pindolol, propranolol and sotalol

The recent developments in enantioselective HPLC-separation techniques are impressive and are driven by industrial and academic interests; thus there is for instance a high demand for developing stereoselective assays for chiral drugs in biological fluids. The beta-blocking agents, which possess an amino-propanol- or -ethanol side chain with at least one chiral centre, represent one of the most intensively investigated groups of more than 40 drugs introduced world wide. Seven of the most popular beta-blockers were chosen as representatives: atenolol; betaxolol; carvedilol; metoprolol; pindolol; propranolol; and sotalol, these span the whole range of lipophilicity to hydrophilicity (polarity). Enantioselective HPLC bioassays for these beta-blockers published so far, including techniques based on chiral derivatizing agents (CDAs), chiral stationary phases (CSPs) and chiral mobile phase additives (CMPAs) have been reviewed and documented in the light of general aspects together with pharmacokinetic and pharmacodynamic considerations.

The role of individual human cytochromes P450 in drug metabolism and clinical response

Recent advances in the study of human cytochromes P450 by protein purification, molecular cloning techniques and analysis of polymorphisms has led to increased understanding of the role of the various forms in the metabolism of clinically important drugs. In particular, the substrate specificity of one form, CYP2D6, is well established. CYP2D6 shows polymorphism, with 5-10% of Caucasians (poor metabolizers) not expressing this enzyme. The molecular basis of this deficiency is now well understood and methods for the detection of poor metabolizers are discussed, as well as the effect of the polymorphism on drug metabolism. Substrate specificities and possible polymorphisms in other cytochromes P450 are also discussed.

Advances in the diagnosis of adverse drug reactions

The clinician plays a key role in the detection and diagnosis of adverse drug reactions (ADRs). The diagnosis of ADRs, however, is a complex task. In the past, there were no systematically applied diagnostic criteria for ADRs, no formal methods of case analysis, no standardized epidemiologic approaches, and a limited knowledge of mechanisms. This resulted in the overdiagnosis of ADRs, which has negative consequences such as limiting treatment options. Recently, there have been various improvements in the diagnosis of ADRs, such as the development of standardized decision aids and of in vitro diagnostic tests. This article briefly reviews some of this knowledge, discusses the role of in vivo and in vitro rechallenge, and summarizes a probabilistic approach for collecting relevant information and diagnosing ADRs. The intention is to increase awareness of the different approaches for diagnosing ADRs as well as to stimulate researchers to continue to collect pharmacoepidemiologic information, study the pharmacologic, immunologic, and genetic factors involved in the pathogenesis of drug reactions, and develop and test new diagnostic instruments under various clinical conditions.

Omeprazole drug interaction studies

This review examines the literature on drug interactions with omeprazole. Different mechanisms have been proposed as potential causes for such interactions. First, the absorption of some drugs might be altered due to the decreased intragastric acidity resulting from omeprazole treatment. There was no effect of omeprazole on the absorption of amoxycillin, bacampicillin and alcohol, while the amount of digoxin and nifedipine absorbed was increased by 10 and 21%, respectively, both increases probably being of no clinical significance. Secondly, the metabolism of high clearance drugs might be altered by changes in liver blood flow, although that is not affected by omeprazole, as indicated by the unchanged elimination of indocyanine green. In addition, the clearance of intravenously administered lidocaine (lignocaine) [a high clearance drug] was unaffected by omeprazole, further indicating that the latter does not alter liver blood flow. Thirdly, since omeprazole is a substituted benzimidazole, it might have the potential to interfere with the metabolism of other drugs by altering the activity of drug metabolising enzymes in the cytochrome P450 system, through either induction or inhibition. There is no indication of induction of this enzyme system in any interaction study with omeprazole. As regards inhibition, on the other hand, there is now considerable information available which indicates that omeprazole has the potential to partly inhibit the metabolism of drugs metabolised to a great extent by the cytochrome P450 enzyme subfamily IIC (diazepam, phenytoin), but not of those metabolised by subfamilies IA (caffeine, theophylline), IID (metoprolol, propranolol) and IIIA (cyclosporin, lidocaine, quinidine). Since relatively few drugs are metabolised mainly by IIC compared with IID and IIIA, the potential for omeprazole to interfere with the metabolism of other drugs appears to be limited.

The role of lipophilicity in the inhibition of polymorphic cytochrome P450IID6 oxidation by beta-blocking agents in vitro

The importance of lipophilicity as a determinant of the affinity of beta-adrenoceptor blocking agents for a specific human hepatic monooxygenase--cytochrome P450IID6 (responsible for the debrisoquine-type of oxidation polymorphism)--was investigated in vitro by estimating the inhibition constants of a series of compounds in a microsomal system with monitoring of the kinetics of dextromethorphan O-demethylation. Lipophilicity is a key predictor of the affinity of beta-blocking drugs for cytochrome P450IID6 and of their potential to cause specific competitive drug interactions, but more complex structural factors appear to be important as well. A high lipophilicity is also a necessary, but not a sufficient condition for these compounds to be metabolized by cytochrome P450IID6.

Genetic polymorphism of sparteine/debrisoquine oxidation: a reappraisal

Polymorphic oxidation of the sparteine/debrisoquine-type has been shown to account for much of the interindividual variation in the metabolism, pharmacokinetics and pharmacodynamics of an increasing number of drugs, including some antiarrhythmic, antidepressant and beta-adrenoceptor antagonist agents. Impaired hydroxylation of these drugs results from the absence of the enzyme cytochrome P450IID6 in the livers of poor metabolisers, who constitute 6% to 10% of Caucasian populations. The clinical importance of the phenomenon has to be explored further and for most sparteine/debrisoquine-related substrates there is a need for controlled prospective studies to define the consequences to the patient of impaired or enhanced drug oxidation.

The genetic polymorphism of debrisoquine/sparteine metabolism--clinical aspects

It has been established that the metabolism of more than twenty drugs, including antiarrhythmics, beta-adrenoceptor antagonists, antidepressants, opiates and neuroleptics is catalyzed by cytochrome P-450dbl. The activity of this P-450 isozyme is under genetic rather than environmental control. This article discusses the therapeutic implications for each of the classes of drugs affected by this genetic polymorphism in drug metabolism. Not only are the problems associated with poor metabolizers who are unable to metabolize the compounds discussed, but it is also emphasized that it is difficult to attain therapeutic plasma concentrations for some drugs in high activity extensive metabolizers.